Nuclear design for mixed moderator PWR

Progress in Nuclear Energy, Vol. 32, No. 314. pp. 533-537, 1998 0 1997 Published by Elsevier Science Ltd Printed in Great Britain 0149-1970/98 $19.00 + 0.00

PII: s0149-1970(97)00040-1

NUCLEAR DESIGN FOR MIXED MODERATOR

H.TOCHIHARA,

Y.KOMANO,

M.ISHIDA,

PWR

K.NARUKAWA,

M.UMENO

Mitsubishi Heavy Industries, Ltd. 3-1 Minatomirai 3-chome, Nishi-ku, Yokohama 220-84 JAPAN

ABSTRACT

The MPWR uses basically the same hardware as a PWR, only the moderator is changed. By varying the moderator from light water to heavy water or a mixture of two, better resource (U,Pu) utilization can be feasible in accordance with the capacity of uranium resource and the Pu supply and demand balance. In this study, the feasibility of MPWR core for an ultra-long cycle of four years is analyzed to achieve high plant availability. 8 1997 Published by Elsevier Science Ltd INTRODUCTION The spectral shift control using variable heavy water fraction in the moderator was proposed in the 1960’s in USA (Edlund.1964). The basic feasibility on the small scale was demonstrated by Vulcain experiment in the Belgium BR-3 reactor (Storrer and Rigg ,1964). Better resource utilization was verified in several investigations (Uotinen et al., 1977; Hellens et al., 1978; Ronen and Fahima, 1984 ), but it is not competitive mainly due to higher capital costs, heavy water cost compared with current Pressurized Water Reactors (PWR). This concept may become a very attractive option if the uranium price increase substantially (Sider and Matzie, 1979). On the other hand, in Japan, the Light Water Reactors (LWR) will continue to play the leading role in the power generation until the introduction of Liquid Metal Fast Breeder Reactor (LMFBR). In addition, Japan opts to recycle the recovered Pu from the spent fuel because Pu is a precious energy resource and this recycling is based on having no Pu stockpile from the non-proliferation viewpoint. Therefore, we propose the use of heavy water concept in combination with the variable operation mode for a flexible Pu utilization in accordance with the Pu supply and demand balance until the introduction of LMFBR. The Mixed Moderator PWR( MPWR) uses basically the same proven PWR hardware, only the moderator can be widely changed from light water to heavy water or a mixture of two. This variation of the moderator implies neutron spectrum change allowing better utilization of resources (U, Pu) in accordance with the resources supply and demand balance. In other words, the introduction of the MPWR, in which the amount of required Pu can be readily changed, makes it easier to control the Pu supply and demand balance and to use the multiple recycled Pu. In accordance with the uranium resources, the efficient use of uranium can be also feasible. First, this paper describes the basic concept of the MPWR operation and then, focuses on the nuclear design of an ultra-long cycle length MPWR, in which the excess of the core reactivity is controlled by the heavy water fraction in the moderator instead of by boron as in current PWRs. The ultra-long cycle achieves high plant availability, which can compensate the higher capital cost of MPWR compared with the current PWRs.

THE CONCEPT OF THE MPWR In this study, a PWR with a gross electric output of about 1420MWe is chosen as a reference plant (Takakuwa et al., 1995). The moderator of the MPWR could be light water, heavy water or a combination of two. Two modes 533

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of operation are possible in the MPWR. The first one keeps the heavy water fraction in the moderator constant during one cycle and uses the cllrrent boron chemical shim to control the surplus core reactivity (mode-a). A parametric study for several heavy water concentrations was performed. The reactivity decreases when the heavy water fraction in the moderator is increased due to the reduction of the slowing down effect of the moderator. So the required Plutonium fissile (Pu’) content musf be increased when the fraction of heavy water in the core. is high in order to achieve the same cycle lengtt The use of heavy water in the moderator shifts the neutron spectrum to higher energy levels and improves the conversion of fertile to fissile material. So, a high heavy water fraction can be used to reduce the amount of Pu to be consumed, which is defined as the difference between the loaded Pu’and the discharged Pu’inventories. On the other hands, a low heavy water fraction or a completely light water moderator can be used to increase the consumed amount of Pu. The use of the heavy water also improves the discharged Pu’isotopic composition, so the multiple recycling lf Pu becomes easier. Further improvements in the conversion ratio could be possible by reducing the moderator to fuel volume ratio, and by adopting axial and/or radial blankets. The second method controls the excess reactivity during normal operation by displacing part of the light +vater by heavy water instead of using the current boron chemical shim( mode-b). Fig.1 shows how the critical heavy aster fraction changes in the case of uranium fuel, and for comparison purposes, the change of critical boron concentration for current operating methods is also shown in the same figure. The required uranium enrichment is smaller than the current PWR because of the high conversion ratio, as shown in Fig.2. For MOX fuel, the clitical heavy water fraction and the critical boron concentration changes as shown in Fig.3. Because of the improvement of conversion ratio shown in Fig.4, it is possible to achieve the same cycle length with lower Pu’ content than the current PWR.

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Nuclear design for mixed moderator PWR

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MOX Fuel

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The above improved utilization of resources can be achieved due to the elimination of neutron losses caused by parasitic absorption in the moderator and the shift of the neutron spectrum to higher energies. While the conversion ratio of the MPWR decreases with bumup mainly because of the softer neutron spectrum due to the reduction of the heavy water, the conversion ratio of a PWR increases with bumup due to the reduction of the parasitic neutron absorption in boron. As a results, the heavy water control improves the fuel performance compared with the current PWR. Thus, the required Pu’ content changes widely for the same cycle length according to the heavy water concentration during operation and the operation mode. The discharged Pu composition is also improved, so the easier recycle Pu utilization can be feasible. Therefore, better resources (U,Pu) utilization can be feasible according with the capacity of natural uranium resources and the Pu supply and demand balance.

SPECTRAL SHIFT FOR A LONG CYCLE OPERATION In this section, the ultra-long cycle operation is presented using mode-b concept to achieve a high plant availability and to compensate higher cost of MPWR due to higher capital cost, heavy water cost, maintenance and operation. The 100% MOX core with a light water moderator has been shown to be feasible in the APWR without any

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problems (Tochihara et al., 1995; Komano et al., 1996 ; Ishida et al., 1996). The cycle length for a full MOX core can be extended more easily than for a uranium core because of the more negative moderator temper,lture coefficient (MTC) at the beginning of the cycle (BOC) and the flatter power distribution. So, the nuclear design feasibility of a two year cycle has been demonstrated for 100% MOX APWR core operation , however ultra-long cycle length, such as a four year cycle, becomes difficult with the current chemical shim because the MTC at 130C becomes positive due to the high required Pu’ content and high critical boron concentration. The use of current burnable absorbers is not effective enough for 100% MOX core because of the harder neutron spectrum. However. the cycle length for a MPWR using the spectral shift operation changing the heavy water to light water ratio dt ring one cycle can be extended easily. A 1420MWe MPWR full MOX core with a cycle length of four year: are analyzed in this section. The MPWR core is composed of 257 17x17 type fuel assemblies. The fuel assemblies and the fuel rod dimensions are the same as for the PWR, so the moderator to fuel volume ratio is kept constant in this study. The four year cycle improves the plant availability and it can be achieved by a single batch reload of about 8wt%Pu’ fuel MOX assemblies. The change of the heavy water fraction with cycle bumup is shown in the Fig.5. For comparison purposes, the boron concentration variations for the PWR, full MOX core with the current cycle length of 13.5EFPM using three batch reloads is shown in the same figure. At the BOC the moderator is almost 70% heavy water and at EOC it has become into almost all light water. The spectrum is shifted to higher energies at the BOC, so higher conversion ratio than the PWR can be achieve as shown in Fig.6.

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Nuclear design for mixed moderator PWR The core physics parameters at the EOC are quite similar to the PWR. because the moderator is almost all light water, but not at BOC. The MPWR has negative MTC value at BOC, hot zero power conditions. The MPWR does not use soluble boron during normal operation, but boron injection is to be used to handle transients. The boron worth decreases due to harder neutron spectrum by the presence of the heavy water, but it can be compensated by using enriched B” instead of the current natural B. The control rod worth at the BOC is slightly smaller for Ag-InCd, but it is larger when the 90%enriched B” is used as the control absorber. Therefore, the MPWR with an ultralong cycle is shown feasible from the nuclear design viewpoint. Finally, it is important to point out that MPWR core is very simple because it can operate with a single Pu content per assembly and with no requirement for any burnable absorbers.

CONCLUSIONS The MPWR, which uses basically proven PWR hardware, can make use of Pu a flexible way to suit the Pu supply and demand balance by changing the heavy water fraction in the moderator. An ultra-long cycle of four years is feasible by controlling the excess core. reactivity by changing the heavy water fraction in the moderator.

ACKNOWLEDGMENT The authors wish to thank Dr. Nobuo Fukumura of PNC for his valuable suggestions and discussions during this study.

REFERENCES Edlund MC (1964). Developments in Spectral Shift Reactors, Proc. 3rd U.N. Conference Peaceful Uses Atomic m, Vo1.6, p.314, New York Stoner J. and Rigg S. (1964), The Vulcain Core Power Experiment, Proc. 3rd U.N. Conference Peaceful Uses Atomic Energy, Vol.1, p.337, New York Uotinen V.0 et al. (1977), Reevaluation of the Spectral Shift Control Reactor, Transactions American Nuclear Societv, Vo1.27. p429 Hellens R.A et al. (1978). Reactor Design Based on the Spectral Shift Control Concept, Transactions American Nuclear Societv, Vol.28, p.574 Ronen Y. and Fahima Y (1984) , Combination of Two Spectral Shift Control Methods for Pressurized Water Reactors with Improved Power Utilization, BTechnoloev. Vol.67, p.46 (1984) Sider F. M. and Matzie R. L. (1979), Plutonium Fuel Cycle in the Spectral Shift Controlled Reactor, Nuclear Technology, Vol.47, p.444 Takakuwa K. et al. (1995). Advanced PWR in Japan, Proc. of ICONE3, Vol.2, pp.663, Kyoto Tochihara H. et al. (1995), Full MOX Core Design in Advanced PWR, Proc. of GLOBAL’95, Vo1.2, ppl401 ,Versailles Komano Y. et al. (1996). Design Features for full MOX Operation in APWR, Proc. of ICONE-4, Vo1.2, pp4753,New Orleans Ishida M. et al. (1996). Improved Full MOX Operation for an Advanced PM, Proc. of the 10thPacific Basin Nuclear Conference, Vol. 1, pp690, Kobe

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